This invention relates to novel polymer blends formed from a rubbery polymer and a major proportion of a glassy thermoplastic polymer such as polystyrene (PS), poly-methylstyrene, polypara-methylstyrene, polymethylmethacrylate, (PMMA), polyvinylchloride, polyethyleneterephthalate, poly(styrene-co-acrylonitrile), poly (styrene-co-methyl methacrylate), polycarbonates and the like.
Prior to the present invention, glassy polymers have been prepared and used in the production of articles formed by molding such as compression molding or injection molding. It is also known to incorporate into glassy polymers a rubbery polymer in order to improve the impact strength of the resultant material. Typically, the polymer components of the blend are mechanically worked together by heated compound rolls and the like. Alternatively, coagulated emulsion latices of the glassy polymer and the rubbery polymer are mixed together to form resultant polymer blends. Complex phase inversion graft copolymerizations are also employed. These polymers contain between about 5 and about 45 weight percent of the rubbery polymer with the remainder being the polymer formed from the glassy compound. Such polymers are disclosed for example in U.S. Pat. No. 3,090,767 and U.S. Pat. No. 3,458,602. While these blends have improved toughness as compared to the pure glassy polymer, it would be desirable to improve the impact strength and toughness of this class of polymer in order to expand the environments within which they can be utilized satisfactorily, particularly in environments where articles produced from the blends are subjected to stress and strain forces. In addition, presently available polymer blends are formed by a process comprising the steps of bulk and emulsion polymerization. This process is time consuming and expensive.
Many atactic flexible chain glassy polymers such as polystyrene, PMMA and their various modifications which have very attractive properties of stiffness and appearance, and are relatively easily processed into finished products, however, suffer from brittleness in unoriented form. While moderate levels of uni-axial or bi-axial orientation can rectify this imbalance, both types of orientation processing limit the range of shapes that can be manufactured and restrict the applicability of manufacturing processes such as injection molding.
Alternative approaches to the alleviation of this brittleness problem have included blending with other less flexible chain homopolymers that are known to undergo plastic flow in tension rather than exhibit crazing. The most widely preferred chain polymers, however, is rubber modification by incorporation of a substantial volume fraction (typically 0.1) of an elastomeric component in the form of block or graft polymer. The elastomer undergoes phase separation, while maintaining a high quality interface between the homopolymer and the precipitated compliant particle phase.
The inelastic strain in flexible chain glassy homopolymers deformed in tension is produced by the dilational plasticity of crazes, initiated from surface imperfections, and the brittleness results from the premature fracture of such crazes when the latter, during their growth, encounter inorganic particulate impurities. Thus, the root cause of the brittleness in question is the fracture of craze matter, initiated from the supercritical flaws that result when a growing craze encounters a poorly adhering inorganic dust particle, typically of micron dimensions. This undesirable response has a number of important ingredients. The surface imperfections on the average have a relatively low potency to initiate crazes. They can provide, at best, plastic response only in a surface layer. For additional plastic extension the crazes must spread under the prevailing tensile stress into the elastic interior regions of the stressed part at rates governed by the kinetics of craze matter production at the borders between the initiated crazes and the solid homopolymer. The combination of the low volume density of surface initiated crazes and their growth kinetics is such that the rates of deformation imposed by a testing machine or an impact require a high tensile stress acting across the crazes to match the dilatational craze strain rate against the imposed rates. Finally, the fracture toughness of craze matter is insufficient to cope with the flaws introduced by the micron size poorly adhering inorganix dust particles present in the solid polymer, when these particles become incorporated into a growing craze. The elimination of the dust particles which initiate the final craze fracture is impractical and is, at best, only marginally effective. Thus, not withstanding the many explanations found in the literature, the principle reason for the effectiveness of high impact polystyrene (HIPS) or acrylonitrile-butadiene-styrene (ABS) polymers in toughening, is the substantial lowering of the craze flow stresses (by more than a factor of two) that results from the heterogeneous rubbery particles which they contain. The principle function of these particles of adequate compliance and size is to initiate a much larger density of active crazes throughout the volume of the solid polymer than what can be generated from surface imperfections. Within the restriction of the normal craze growth kinetics this then permits the matching of the plastic craze strain rate to the imposed deformation rates under a considerably lower tensile stress for which the flaws due to most the usual dust particles now become sub-critical and the polymer survives long enough to undergo substantially larger strains to fracture.
Therefore; it would be desirable to produce polymer blends which retain the stiffness of the glossy component while exhibiting substantially reduced craze growth stresses, thereby increasing toughness substantially; it would also be desirable to provide a process for preparing polymer blends which reduces the number of process steps required for producing the blend.